1. Field of the Invention
The present invention relates to the measurement of lung water content, and more particularly, to apparatus and methods for passive, noninvasive determination of lung water content and distribution through the measurement of electromagnetic radiation emitted naturally by the animal body.
2. The Prior Art
It is well known in the medical arts that the change in lung water content is an important and useful parameter in medical diagnostics. Changes in the water content of the lung sre symptomatic of numerous medical and surgical abnormalities. For example, in almost all pulmonary abnormalities occuring in patients with heart disease or critical burns, changes in lung water content or distribution are detectable. Thus, the monitoring of such changes in the lung water content can facilitate the early identification of such abnormalities.
Although a means for the accurate measurement of changes in lung water content could be an extremely valuable diagnostic tool, those devices and methods which are currently known and used by medical practitioners have many deficiencies. Presently known methods are incapable of sensitive measurement, overly complicated and elaborate, and time-consuming to use. In addition, these methods require highly skilled technicians for their operation, involve exposing the patient to possibly dangerous radiation, and are not suitable for continuous monitoring which is desirable for most applications and is essential for critically ill patients.
Historically, one widely used method for attempting to identify changes in lung water content has been to monitor the lung with the chest radiograph. However, this method is very insensitive, often requiring a doubling of total lung water content before changes can be detacted on the x-ray photographs. As a result, this radiographic method is only reliable as a basis for diagnosis in the most advanced cases. Furthermore, because of the hazardous effects of long term or excessive exposure to x-rays, this method cannot be used for continuously monitoring the patient.
Another technique which as been used for detecting changes in lung water content is electrical impedance ("EI") plethysmography. The basic idea behind this low-frequency electromagnetic method is that when a low frequency voltage is applied across an isolated lung, the measured current is a result of the movement of ions in extracellular water. Theoretically, since air constitutes the major component of the lung's volume, the resistance of the lung should be high so that this method should be sensitive to changes in blood and extracellular volumes. However, disappointing in vivo measurements have been obtained from this method due to the short-circuiting effect of the more conductive surrounding tissues, such as the medistinum and the chest wall. This short circuiting effect significantly reduces the sensitivity of the method, with the result that rather large changes in lung water content are again necessary before any clear detection of a change is possible.
Subsequent improvements of these methods include the addition of guarded electrodes and focusing electrode bridges in conjunction with EI plethysmography. Although these improvements have given the EI plethysmography method an improved sensitivity, the improvement has not been significant and has only resulted in a sensitivity similar to that obtained from the chest radiograph.
Therefore, techniques such as these for measuring the changes in lung water content have not proved acceptable for most diagnostic purposes which require sensitive identification of small changes in the quantity of water contained in the lung.
Several recent attempts to provide more sensitive measurements of changes in lung water content have involved the use of penetrating microwave radiation. Such methods are based upon the fact that variations in lung water content cause a change in the permittivity and the conductivity of the lung tissue, thereby changing the absorption characteristic of the transmitted microwave signals.
One of these methods utilizes electromagnetic waves which are attenuated as they travel through the body. Although this method has been shown to have much greater sensitivity than those methods previously discussed, it has been found to be difficult to use this method with actual patients. This method requires use of both a transmitter and a receiver, each positioned upon opposite sides of a body so that signals can be transmitted therebetween; the lung water content is determined by the measured changes in the magnitude and phase of the transmitted signal.
The problems with this method result from the necessity for accurate alignment of the transmitter and receiver, which alignment should be maintained during the entire course of the series of measurements. Thus, during such monitoring, simple movements by the patient (such as changing position in bed, moving the arms or legs, and stretching) cause alignment problems between the transmitter and receiver such that inaccurate and unreliable measurements result. In addition, since this method involves the transmission of microwaves through a human body, the possible deleterious effects on the patient's health preclude its application in a continuous monitoring technique.
A further problem experienced with prior art methods and systems directed to the use of microwaves for monitoring biological bodies is associated with attempts to reduce the size and weight of the microwave antenna while providing for impedance matching between the antenna and the biological body. One device which is somewhat reduced in size over other types of microwave antennas is the ridged waveguide. This type of waveguide provides a means for lowering the cutoff frequency in order to monitor at lower operating frequencies or, alternatively, for using a smaller aperture size.
To further reduce the required size of the aperture or to permit still lower frequency operation, dielectric loading is employed. Typically, dielectric loading is accomplished by securing dielectric material in the aperture of the waveguide. One adverse result of adding dielectric is that the weight of the antenna increases, thus making it more difficult to secure the antenna in an immobile position on the patient, and making the antenna more uncomfortable for the patient to wear. Furthermore, because of the increased mass resulting from dielectric loading, it becomes more difficult to reach and maintain an antenna temperature level which substantially equals the temperature of the adjacent biological body. It would be an improvement in the art to provide small, lightweight, dielectric free microwave antennas which are adjustable to approximate the impedance of the biological body in an air medium and which are readily adaptable for use in passive, non-invasive monitoring.
In light of problems such as these, which have been encountered in the techniques heretofore known in the art, it would be a substantial improvement to provide a passive, noninvasive system for measuring changes in lung water content which is both sensitive and accurate so as to identify even small changes in lung water content. It would be even more significant if this system were clinically safe for use in long term continuous monitoring of lung water content within the human body.